We know that general formula for the line is y=ax+b, we know that a=1/2. To get b we can subsittute x=10 and y=-1 (from the second point)
-1=1/2*10+b
-1=5+b /-5
-6=b
We know that our line is y=1/2x-6
Now we can substitute x=6 (from first point) and find value of y
y=1/2*6-6
y=3-6
y=-3 - its the answer